A. Field of the Invention
The field of the present invention relates generally to propulsion systems that utilize charged particles to generate the propulsive forces to propel an object. More particularly, the present invention relates to ion thrusters that are adapted for use in the Earth's atmosphere. Even more particularly the present invention relates electrically powered, air-breathing ion thrusters capable of operating in low-Earth atmosphere.
B. Background
Propulsion systems that are capable of propelling a vehicle through the atmosphere that do not require a large quantity of fuel to be carried by the vehicle for its own consumption have long been desired. As is well known, a significant portion of the overall weight of a vehicle that travels through the atmosphere can be the fuel necessary to propel the vehicle. This generally results in a balance being chosen between the weight of non-fuel materials that can be carried by the vehicle or the distance the vehicle can travel, with a greater amount of materials reducing the quantity of fuel available, and therefore the distance the vehicle can travel, and the additional fuel for greater distance limiting the weight of materials that can be carried. To overcome this problem, propulsion systems with greater efficiency have been developed, particularly those that utilize readily available natural resources as the fuel, such as solar powered aircraft that utilize solar cell technology to power the vehicle's engine. Although other types of fuel systems have been developed or suggested for atmospheric vehicles, including various magnetically or nuclear powered propulsion systems, limitations due to efficiency and safety concerns have generally prevented full acceptance of such systems.
Propulsion systems utilizing ion engines for use in high atmosphere and space vehicles, including those which are configured for travel in the atmosphere of other planets, have been developed and somewhat successfully utilized for many years. The typical ion engine propulsion systems requires a propellant such as mercury, xenon, argon or cesium for ionization and operates as a Hall effect thruster. Electrons emitted by a cathode are directed into a discharge chamber where propellant is introduced to collide with the electrons in order to create positively charged ions that are rapidly expelled from the discharge chamber to generate the engine's thrust. An example of such a system is disclosed in U.S. Pat. No. 4,838,021 to Beattie, which describes an ion thruster having an ionization chamber formed by a cylindrical metallic conductive sidewall that functions as the anode in which propellant gas, such as xenon, is ionized by electrons emitted by a cathode to produce a plasma that expels an ion beam to create thrust. U.S. Pat. No. 3,952,228 to Reader, et al, describes a cylindrical shell which defines a chamber in which an ionizable propellant, such as argon, is introduced. Disposed symmetrically within the shell is a cylindrical anode, which has a cathode centrally positioned therein. An apertured screen and an aligned apertured grid at the open end of the cylinder draw ions along a beam path to create thrust. A major limitation to such propellant systems, as with conventional fuel powered vehicles, is the need to carry sufficient propellant to achieve the desired operation of the vehicle. Longer flight or other engine operation time requires the vehicle to carry larger quantities of propellant, which increases the weight of the vehicle and thrust requirements for the engine, which then requires a larger and generally heavier engine that needs even more propellant to effectively operate. As a result, there has been a need for vehicle propulsion systems utilizing ion powered engines that do not require the use of stored and carried propellant.
For operation in the Earth or other Earth-like atmospheres, there have been developed air-breathing ion engines that utilize ambient atmospheric gas, which is sufficiently ionizable, as the propellant. These engines draw in the atmospheric gas and ionize a portion of it utilizing cathode devices, instead of having to carry ionizable fuel on the vehicle, to achieve the desired thrust from the rapid discharge of charged ions. Some of these ion engines have been patented. For instance, U.S. Pat. No. 6,834,492 to Hruby, et al. describes an air-breathing electrically powered Hall effect thruster having a thruster duct with an inlet, an exit and a discharge zone therebetween, an electrically charged cathode for emitting electrons, an anode in the discharge zone that attracts the electrons and a magnetic circuit that establishes a radial magnetic field in the discharge zone. The magnetic field creates an impedance to the flow of electrons toward the anode to better ionize the atmospheric gas moving through the discharge zone. This enables ionization of the atmospheric gas and creates an axial electric field in the thruster duct for accelerating the ionized air through the exit to create thrust. U.S. Pat. No. 6,145,298 to Burton, Jr. describes an ion engine propulsion system that utilizes a high voltage power source to ionize a portion of high altitude ambient atmospheric gas to create a negative ionic plasma which bombards and accelerates the remaining atmospheric gas in a focused and directed path to an anode receiver to create thrust for propulsion. The cylindrical cathode is tapered, preferably to a fine point, and the anode is substantially ring-shaped or comprised of a plurality of concentric rings of decreasing diameter that are axially aligned with the tapered cathode. The tapered cathode and ring-shaped anode are disposed in a housing that has an inlet for receiving ambient atmospheric gas and an outlet for discharge. A voltage power source having a negative potential is connected to the cathode and a power positive source is connected to the anode. An electromechanical arrangement is provided to adjust the distance between the cathode and anode.
Despite the foregoing, there exists a need for an improved air-breathing electrostatic ion thruster for use in low-Earth atmosphere. The preferred ion thruster should utilize ambient atmospheric gases as the propellant so as to eliminate the need for the vehicle to store and carry a sufficient quantity of propellant. The preferred ion thruster should have a housing with an electrically conductive inner surface that defines a ionization chamber in which is disposed an electrically charged inner electrode and which has electrically charged screen electrodes at its inlet and outlet to repel, attract and accelerate ions so as to generate thrust due to the ionization of the atmospheric gas. The preferred ion thruster should be configured to be relatively simple to manufacture and operate and will provide long and reliable operation.